Currently, in accordance with IETF RFC2543 SIP standards, Session Initiation Protocol (SIP) addresses are employed to identify respective users within a single domain, and the operation processes of SIP components are described with the position information of all the users considered as being able to be known in a SIP server (proxy server, or redirect server) within the domain. That is, it is assumed that the current position information of all the users can be known in the SIP server.
There are proposed three schemes as methods of realizing the above standards. The first one is implemented in such a way that a single SIP server and a single user position information registrar are provided in a domain. The second one is implemented in such a way that a single registrar is provided at the center of a plurality of SIP servers. The third one is implemented in such a way that a plurality of SIP servers arranged in a tree structure are each provided with a registrar.
The first scheme is advantageous in that only one SIP server and only one registrar are provided, so its structure is simple and its implementation is easy. However, the first scheme is disadvantageous in that users that can be accommodated in the domain are confined to a relatively small number.
The second centralization scheme is disadvantageous in that the requests of all users are concentrated to a single registrar, so the processing speed of the entire system is decreased.
The third distribution scheme relieves the shortcomings of the first two schemes described above. The third distribution scheme tends to be applied to the most intra-domain architectures because the scheme is capable of being expanded to accommodate a relatively large number of users and the concentration of the load on a server can be prevented.
In this case, in order to register user position information in such a distributed intra-domain, the synchronization of information held by a variety of registrars must be preconditioned. In accordance with a conventional synchronization technique in the distributed intra-domain, user position information is found by means of a simple timer function. Accordingly, the conventional synchronization technique is inaccurate and inefficient. In this prior art, user registration information is stored in a database for a certain period of time, e.g., one hour, and the user registration information is cancelled after an elapse of a certain period of time, e.g., one hour. However, this prior art does not provide a reliable scheme of synchronizing user registration information.
Accordingly, there have been pursued reliable techniques of accurately and efficiently synchronizing the user registration information generated in the registrars of the distributed intra-domain.